The invention relates to a device for measuring a mass flow, particularly of a bulk material flow whereby the Coriolis measuring principle is employed.
For a precise ascertaining of the weight of material flows the effective inertia force (Coriolis force) is utilized for the mass determination. The Coriolis force occurs at the coupling of a moved mass particle with a rotating reference body. For this purpose the material flow is supplied to a disk provided with radial guide vanes, whereby the bulk material is centrifuged substantially perpendicularly to the rotational axis of the disk. The torque moment of the disk rotating with a constant r.p.m. varies in accordance with the Coriolis force, whereby the torque moment change is proportional.
Such a measuring device for the continuous ascertaining of the weight of material flows is known from German Patent Publication DE-OS 3,346,145. In that measuring device an impeller rotating with a constant r.p.m. is driven by an electric motor arranged directly above. The motor is supported in a stationary housing in the manner of a pendulum. A lever arm is attached to the drive motor by means of which the rotatably mounted motor supports itself on a force sensor which is connected with the housing. Thereby, the reaction torque moment of the occurring torquing of the motor housing is transmitted to the force sensor. The sensed force is converted into a torque moment by taking into account the lever arm, whereby the torque moment is precisely proportional to the mass throughput. In that apparatus the motor is suspended like a pendulum and is guided relative to the stationary housing by ball bearings which can falsify the torque moment measurement by their friction. Even if one ascertains this bearing friction by a measurement made during an empty run and then takes the measurement into account, different bearing frictions occur under load operating conditions so that with a motor supported at one side, due to a non-uniform lubrication on the bearing walls, frictions occur that cannot be determined and which falsify the measuring result.
A torque moment measuring device with a torsion articulation is known from European Patent Publication EP 0,590,187 A1 for avoiding such non-uniform friction effects. The torsion articulation introduces the drive torque moment through three vertically arranged leaf springs into at least two separate force sensor elements, whereby the drive torque moment of the motor always bears symmetrically to the rotational axis onto the two force measuring elements. However, for this purpose at least two force measuring elements are required which increase the apparatus effort and expense of the measuring device.
Therefore, it is the object of the invention to improve a measuring device of the type mentioned above in such a way that with simple means a high measuring precision is achievable.
The invention employs the Coriolis measuring principle in an apparatus which is characterized by the combination of the following features, an impeller, a drive motor for driving impeller with a constant r.p.m., a bulk material supply for feeding said flow of bulk material to said impeller for radially detouring said bulk material flow, a force measuring element, a torque moment transmitting device for transmitting a drive torque moment applied by said motor to said impeller, to said force measuring element, said torque moment transmitting device comprising a rotatable bearing, at least two horizontal angularly spaced leaf spring elements supported by said rotational bearing for rotation about a central axis defined by said rotatable bearing, said at least two horizontal angularly spaced leaf spring elements crossing each other in said central axis for transmitting said torque moment to said force measuring element, said leaf spring elements being flexible in response to bending loads effective in a rotation direction about said central axis and stiff against bending in a leaf spring length direction.
The invention has the advantage that due to the crosswise arranged leaf spring elements, the measuring device always remains fixed on a predetermined rotational axis. Thus, the lever arm length for bearing on a stationary force measuring device cannot vary so that a good zero point stability and measuring value constant is achievable. By these features simultaneously a good insensitivity to temperatures is achieved because the heat expansions can become effective on the lever arm length only in a negligible manner.
According to a special type of embodiment of the invention with an axial ball or toe bearing, it is an advantage that thereby the weight of the drive train is transferrable substantially with low friction onto a stationary housing section, whereby friction caused measuring errors have become substantially avoidable. The drive train includes the impeller, the drive shaft, and the motor. Due to the fact that in such an embodiment the rotation bearing does not have to take up any axial forces, hardly any force shunting effect takes place so that with such a measuring device even small conveying volumes are measurable in an advantageous way and manner. In such an axial support it is also advantageous that the support can be protected in a simple manner against contaminations so that precision is assured over long periods of time.
In a further advantageous embodiment of the invention a friction free horizontal mounting of the drive shaft is achieved with the aid of a fluid bearing in the lead through area so that here friction related measuring errors are also avoided. Simultaneously such a fluid or air bearing assures an excellent sealing to the bulk material space so that the measuring device is advantageously also operable with excess pressure without any apprehension that a contamination and friction increase of the bearing might occur.